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Spring rate change by cutting springs??

Discussion in 'Downhill & Freeride' started by WBC, Mar 26, 2012.

?

Why is Sam B so gnarbar?

Poll closed Apr 5, 2012.
  1. Sam B uses a chainsaw on pinatas filled with airplane shots

    61.5%
  2. Sam B rides around with a chariot full of hardcore VHS pornography

    69.2%
Multiple votes are allowed.
  1. WBC New Member

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    Does anyone here know how to calculate the change in spring rate that occurs when you chop a spring?

    The reason why I ask is that I have a pike 454 dual air U-turn that has felt horrible since day 1, and after dealing with it for 3 years, I want to go coil. I have a couple boxxer springs that I intend to cut down since I don't think RockShox makes a spring stiff enough for me. The red boxxer spring is 50 in/lbs, right? Could I get 70 in/lbs out of it? Do I need to injinere an air/coil combo to get close to that? I am 220lbs, and am looking for no more than 20% sag. And I want to spend less than $10 on this.

    Every helps would most be triumphantly appreciate
    #1 Mar 26, 2012   
  2. jonKranked Press Button, Receive Stupid

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    wow, i can't believe i gave a serious response to this kind of injinereing request. :facepalm:
    Last edited: Mar 26, 2012
    #2 Mar 26, 2012   
  3. ridiculous New Member

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    holy damn. Get on pinkbike and offer ten bucks for a used but correct spring in the BUY/SELL area.
    #3 Mar 26, 2012   
  4. WBC New Member

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    Nah, spring rates increase when you cut the spring. I don't know why this is, as I got a BA not a BS, but I do know it's true. I also DO intend to preload it like crazy, and I believe that RockShox never made a pike spring stiff enough. I had a coil pike before this, and I tried "the stiffest spring [RockShox] make" and never got the fork to have less than 40% sag. I do not care about damage to the inside of the fork, and I intend to run stainless washers top and bottom to keep the spring from pushing through the bottom of spring seat.

    If this fork last through the summer, I'll be ecstatic. All the ano is worn off the stanchions, the lowers have fat gouges and there is cracking around the maxle, and I have applied RTV to the MoCo cartridge to keep it from puking oil all over the bed of my truck when the bike is at an angle.
    #4 Mar 26, 2012   
  5. jonKranked Press Button, Receive Stupid

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    so you're after a darwin award?
    #5 Mar 26, 2012   
  6. Shepherdwong Member

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    I believe that although coiled, springs act like diving boards. Shorten them and it's like rolling the wheel up, they get springier.
    #6 Mar 26, 2012   
  7. jonKranked Press Button, Receive Stupid

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    #7 Mar 26, 2012   
  8. IH8Rice I'm Mr. Negative! I Fail!

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    youd be wrong
    #8 Mar 26, 2012   
  9. jonKranked Press Button, Receive Stupid

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    whether or not something is true has no bearing on whether or not people believe it
    #9 Mar 26, 2012   
  10. WBC New Member

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    Ok, well this isn't a trolling thread. I'm going to do this since I can't see putting money into a clapped out pike. You must be the 1%, so watch out or I'll occupy your ass.

    And I've called SRAM direct when I worked in shops and never got satisfaction for this issue. This is why I'm doing it on my own. Really, it's my fault for posting on a US forum and not a Kiwi forum where people believe in using creativity instead of cash to better their riding experience.

    H8rs
    #10 Mar 26, 2012   
  11. jonKranked Press Button, Receive Stupid

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    there's a fine line between "clapped out" and broken.

    If you've got cracks around the maxle as you've indicated, then your fork has clearly crossed the line into "broken" territory. Use at your own peril.
    #11 Mar 26, 2012   
  12. WBC New Member

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    255109_808295033370_25909849_40656648_3600856_n.jpg
    #12 Mar 26, 2012   
  13. jonKranked Press Button, Receive Stupid

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    pic unrelated.
    #13 Mar 26, 2012   
  14. WBC New Member

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    Look at the poll, it is related.
    #14 Mar 26, 2012   
  15. jonKranked Press Button, Receive Stupid

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    halp! i've been sucked into the PBV!! halp!
    #15 Mar 26, 2012   
  16. gemini2k Active Member

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    Well as long as you put spacers so it doesn't rattle, bend the cut end so it doesn't gouge the fork, and you have enough spring travel for the damper so that the spring doesn't "bottom out" and bind up, it COULD work.
    #16 Mar 26, 2012   
  17. zero knowledge Member

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    In theory the spring rate is inversly proportional to the number of active coils. So if you halve the number of active coils, you'll double the spring rate.
    #17 Mar 26, 2012   
  18. no skid marks Active Member

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    Have you thought of putting an elastomer inside a coil spring. Far from ideal or consistent, but probably easier. Or yes, air and coil is great and better than an elastomer. If you have cracks, it's all over, get a new used fork. You pretty much have no choice IMO.
    Cutting a coil would also be fine, just getting the end tidy will be hard.
    #18 Mar 26, 2012   
  19. dilzy New Member

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    Stop riding mountainbikes until you have more than $10? Seriously, cutting springs is fine, BUT!!!!! you need to make sure it has the requisite travel between coils, then you have to heat and bend down the end coil flat, then grind it, then re-temper the whole spring.

    In other words, not what you were going to do. ......have I just been trolled?
    #19 Mar 26, 2012   
  20. baca262 New Member

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    wtf is going on here? :rofl:
    #20 Mar 26, 2012   
  21. jonKranked Press Button, Receive Stupid

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    a meeting of "injineres"
    #21 Mar 26, 2012   
  22. sundaydoug Member

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    #22 Mar 26, 2012   
  23. tacubaya Member

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    You can read about it on chapter 10 of Shigley's Mechanical Engineering Design.

    K (constant of a spring) is equal to (d^4*G)/(8*D^3*Na) where G is the shear modulus of the material, d is wire diameter, D is the mean spring coil diameter and Na is the number of active coils.

    If the spring is squared and grounded at the ends, the Na = Nt -2, where Nt is the total number of coils.

    Hope it helps.
    #23 Mar 26, 2012   
  24. 4130biker Active Member

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    I've run a cut spring in my boxxer for a long time. Grind the end as flat as possible, then use something to stop it from fvcking up the inside of the fork. I don't think it's that big of a deal. Also I agree that the x firm pike coil is still under sprung- for me at 190, but I like my set ups stiff. I still had to run the fork with floodgate and compression almost all the way up for enough support. At that point an air fork would've been better... Or the right spring rate coil.
    #24 Mar 26, 2012   
  25. 4130biker Active Member

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    Ps why not guess and check? It's a stupid spare boxxer spring. I do agree with others that a different old fork would be safer...
    #25 Mar 26, 2012   
  26. no skid marks Active Member

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    If you could find a metal cover for the end of the spring that's fit in the stanchion you could get a away with less finishing. Or get something machined up with your $10 to fit in the end of the cut spring to locate it and step out to cover the top of the spring, Just make sure cut end is the stationary end.
    #26 Mar 26, 2012   
  27. marshalolson Active Member

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    just stick an x-firm pike spring in the air chamber and run 5-10psi +, and you will be fine.
    #27 Mar 27, 2012   
  28. IH8Rice I'm Mr. Negative! I Fail!

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    totally
    #28 Mar 27, 2012   
  29. jonKranked Press Button, Receive Stupid

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    except for the cracked lowers...
    #29 Mar 27, 2012   
  30. WBC New Member

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    I've had a number of bikes disintegrate under me, including a Sunday that sheared at WFO. I've ridden bikes for 15 years, I've been a wrench for eight, I've taken university level engineering courses. A few stress fractures aren't of too much concern, especially around the dropout. JonKranked, did I mention I build the airplanes your family flies on?

    Thanks to those that helped, especially tacubaya (sp?), I was looking for a starting amount so that I could cut the preload spacer and spring about right and then fine tune with washers. I'll use that formula from mi primo.

    And both answers on the poll are correct, I just couldn't get the second photo to load.
    #30 Mar 28, 2012   
  31. jonKranked Press Button, Receive Stupid

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    "its just a flesh wound"




    tumblr_kudj1dFTrj1qzvqipo1_400.png

    come back and try to impress us when you start designing airplanes
    #31 Mar 28, 2012   
  32. kickstand Active Member

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    i'm glad I don't fly much
    #32 Mar 28, 2012   
  33. Huck Banzai New Member

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    This thread broke me, I am go to doctor now.
    #33 Mar 28, 2012   
  34. Tmeyer New Member

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    Can't believe I just read this entire thread.... Must be Friday.

    Props for the Monty Python pic!
    #34 Mar 30, 2012   
  35. DirtyMike Turbo Fluffer

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    Ummmmmm.... Really? College level engieering class's?

    You should understand that the spring in your fork was engineered for a specific length with the coils it has so it works properly in that fork.

    Before cutting the spring maybe it would be better to do some research and find out more about it. Maybe find out if it is proggesivly wound or anything like that. I have no idea, but I work on cars and suspension all day long, went to college and everything, and I know that cuting springs does nothing but cause problems, as does too much or too little preload.

    But hey, its your bike, your life, your safety....... Do the math, figure out how much you want to cut and go for it if your so inclined. I jsut cannot belive ht if I can find springs that are strong enough for me with a pike, that your going to be undersprung with their extra heavy spring....... Just throwing this out there, but maybe its a compression control issue, and not a weak spring?
    #35 Mar 30, 2012   
  36. wood booger New Member

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    Please, no intelligent and or educated thoughts here.
    #36 Mar 31, 2012   
  37. MinorThreat Active Member

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    I got educated much from mtg and others in my Stupid Ti Spring Question thread. After that, I read a lot of other threads in other disciplines (cars, dirt bikes, etc,) that confirmed: shortening the spring increases the rate proportionately.

    As DirtyMike says, the rate is dependent on (in my crude, non-engineer-speak way) all of the coils being there to lend their amount of 'give.' Whatever percent you shorten a spring by, you stiffen it by that percent.

    In my case, I was looking at shortening a 400# 7.5" spring to 6.5" - - taking an inch out - - and grinding a new flat end. BUT, it would have made it a 465# spring then. Now, if I got a 350# 7.5" spring and removed an inch, it would make it a 402.5# spring; right where I wanted to be. (Decided against it because I was also advised against cold-working ti and I didn't want to dump $100+ on an experiment to save money)
    Last edited: Mar 31, 2012
    #37 Mar 31, 2012   
  38. Sam B New Member

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    That's from my birthday!

    The Porn Charriot makes it in a Pat thread. This is some real injenering here. Awesome!





    PS: Also seen in the 2011 Transition Holiday Card. LOL

    And more...
    Last edited: Apr 1, 2012
    #38 Mar 31, 2012   

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